This invention relates generally to earth boring percussion bits. More particularly, the invention relates to retention systems for ensuring that even when broken during use, the percussion bit remains connected to the drill string.
U.S. Pat. No. 5,065,827 discloses a conventional hammer and percussion bit assembly having a percussion bit retention system. The percussion bit retention system includes a retainer which is mounted on the drive sub of the percussion bit assembly. The retainer includes a lower section having an inner chamber and a thread-shaped, inwardly extending projection. The bottom portion of the percussion bit includes a head section, a threaded section and a recess disposed intermediate the head section and the threaded section. The percussion bit is installed by inserting the upper portion of the percussion bit into the retainer until the threaded section of the lower portion abuts the thread-shaped projection of the retainer. The projection is brought into threaded engagement with the threaded section of the retainer and the retainer is rotated until the projection passes completely through the threaded section of the retainer and enters the chamber. If the percussion bit breaks in the shaft portion above the bottom portion of the percussion bit, the projection will be captured within the chamber by the threaded section of the retainer.
The threaded section of such retainers may become fouled during drilling operations, making removal of the percussion bit difficult. Bits may be difficult to install due to corrosion and wear induced degradation of the threaded section of the retainer.